Different herbicide formulations have different properties, and in particular their viscosities may be different. Thus, where herbicide is fed through a restrictor to a distribution element for distribution over the ground, the flow rates through the restrictor will not be the same for two different formulations. Furthermore, the flow rate of herbicide will vary from one piece of equipment to another, even when they are set to give the same nominal flow rate. Additionally, the flow rate of herbicide must be adapted to the nature of the herbicide, the level of infestation of the weeds to be controlled, the width of spraying, and the speed at which the equipment is moved over the ground.
Modern herbicides are highly effective, and very small volumes of undiluted herbicide are often used, rather than the highly diluted formulations which have been used in the past. Consequently, for maximum effectiveness and efficiency, there must be very accurate control of the rate of delivery of the herbicide from the delivery equipment. It is not, in most cases, adequate merely to rely on a rough-and-ready setting of the equipment. Hitherto, equipment has been calibrated to establish accurate flow rates by operating the equipment so that the fluid is delivered into a measuring jar. The equipment is operated for a predetermined time, for example one minute, the volume of fluid delivered into the measuring jar is then measured, and a calculation is performed to determine the actual flow rate of herbicide. The equipment is then adjusted, and the operation repeated, until a desired flow rate is achieved.
The problem with this procedure is that the substantial quantities of herbicide which are delivered into the measuring jar need to be disposed of. Modern herbicides are expensive, and consequently it is wasteful merely to throw it away. Moreover, the herbicide must be disposed of with care if unwanted damage to plants, and dangers to humans and animals, are to be avoided. Safe disposal can be a particular problem to an operator who is working at some distance from an appropriate waste disposal facility.
These problems are particularly acute where the herbicide for distribution is contained in a package which has a small outlet aperture for connection to the equipment, but which has no provision for refilling.
Our British patent application No. 8715222 discloses a calibration vessel which can be fitted releasably to fluid delivery equipment for measuring the delivery rate. At the end of a test, the fluid collected in the vessel can be returned to the original container through the delivery equipment. With this known device, however, the calibration vessel is separate from the delivery equipment, and consequently there is the danger of it becoming mislaid. Also, there is the inconvenience of having to clean the vessel as well as the delivery equipment.